Coexistence of aerobic chemotrophic and anaerobic phototrophic sulfur bacteria under oxygen limitation

The aerobic chemotrophic sulfur bacterium Thiobacillus thioparus T5 and the anaerobic phototrophic sulfur bacterium Thiocapsa roseopersicina M1 were co-cultured in continuously illuminated chemostats at a dilution rate of 0.05 h(-1). Sulfide was the only externally supplied electron donor, and oxygen and carbon dioxide served as electron acceptor and carbon source, respectively. Steady states were obtained with oxygen supplies ranging from non-limiting amounts (1.6 mol O-2 per mol sulfide, resulting in sulfide limitation) to severe limitation (0.65 mol O-2 per mol sulfide). Under sulfide limitation Thiocapsa was competitively excluded by Thiobacillus and washed out. Oxygen/sulfide ratios between 0.65 and 1.6 resulted in stable coexistence. It could be deduced that virtually all sulfide was oxidized by Thiobacillus. The present experiments showed that Thiocapsa is able to grow phototrophically on the partially oxidized products of Thiobacillus. In pure Thiobacillus cultures in steady state extracellular zerovalent sulfur accumulated. in contrast to mixed cultures. This suggests that a soluble form of sulfur at the oxidation state of elemental sulfur is formed by Thiobacillus as intermediate. As a result, under oxygen limitation colorless sulfur bacteria and purple sulfur bacteria do not competitively exclude each other but can coexist. It was shown that its ability to use partially oxidized sulfur compounds, formed under oxygen limiting conditions by Thiobacillus, helps explain the bloom formation of Thiocapsa in marine microbial mats.